Publications

It is important to interpret and evaluate the fracture mechanism of deteriorated concrete structures with rebar corrosion. Monitoring by means of acoustic emission (AE) technique based on the clarified fracture process of the deteriorated concrete is also useful for maintenance scheme. This paper presents an experimental investigation on the flexural failure behavior of RC beams having different weight loss of 0, 5, 10 and 30% due to corrosion. AE was also monitored during the loading tests of the deteriorated concrete beams. As corrosion levels represented by weight loss increased, load carrying capacity of the beams decreased dramatically. In the case of the severest damage having weight loss of 30%, the rebar was eventually broken at the final stage. The mechanical behavior was attributed to the decrease of cross section of rebar and deterioration of bond properties (debonding) between concrete and rebar with corrosion. The deterioration of bond properties also provided the decrease of number of cracks within the beams. Regarding observation of AE activity, the number of AE events in the beams increased with increasing the corrosion levels, especially as it became more intense at the initial loading level before yielding of rebar. The bond deterioration in the beams with corrosion might occur around the rebar at an early loading stage. The AE sensors attached directly onto the rebar detected more AE signals with lower frequency than that of concrete. It was concluded that such detected AE signals with lower frequency appear to be generated by debonding behavior due to corrosion.

Recent lower hybrid current drive (LHCD), and heating (LHH) experiments on JT-60 are reported. The current drive product of neRpIRF approximately 12.5*1019 m-2 MA was achieved at the LH power of approximately 4.5 MW, and the CD efficiency, the energy confinement, the global power balance and the heat load on divertor plates were investigated in high power LHCD plasmas. Nearly steady state H-mode discharges were found during LHCD with two different frequency injections. Sawtooth suppression in NB heated plasmas by LHCD have shown an improvement in confinement near the plasma center. Parametric instabilities in LH heating experiments were significantly reduced by increasing the plasma current, and the stored energy increased linearly with heating power of up to approximately 9 MW at ne approximately 7*1019 m-3 and Ip=2.75 MA. Parametric instabilities near the plasma edge in the ion heating regime were also reduced in peaked density plasmas produced by pellet injection and LH waves increased the central plasma pressure at ne(0) > 1.4*1020 m-3.

本研究では,RC部材の非線形有限要素解析における損傷評価手法の確立を目的として,コンクリートの圧縮破壊が支配的となるせん断圧縮破壊するRC部材の破壊基準としてのひずみ評価指標を検討した.面的なひずみ性状を計測可能なアクリル格子法を用い,損傷領域と損傷度を実験的に評価するとともに,3次元非線形有限要素解析による損傷評価を行った.その結果,平均化ひずみを用いることで要素寸法によらないひずみ分布性状が得られることを示し,ひずみによる損傷領域評価と破壊基準を提案した.また,提案手法を曲げ破壊するRC部材に適用し,その妥当性と適用性を示した.さらに,不静定構造物を対象とした検討の結果,構造全体の耐荷力以前における部材の破壊を評価でき,単一部材のみならず構造物中の部材の損傷評価に対する適用性を示した.

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For the RC members under cyclic loading, flexural cracks commonly propagate in two opposite transverse directions at the zone under high bending moment and possibly form the cracks penetrating through the entire cross sections (pre-crack), which is considered as a crucial factor for flexure-shear failure of member. This paper clarified the influence of imitation pre-crack on the shear failure behavior of RC slender beams by three point bending test. Shear span depth ratio (a/d=3.14, 4.69) and location and thickness of pre-crack were set as main variables. As the important findings, it was revealed that the influence of pre-crack on the shear strength of RC slender beams is relatively small because it does not affect the formation of the critical diagonal crack and the mode of diagonal tension failure. It was also noted that the pre-crack at 2d (the section of pre-crack is 512 mm, which is twice of the effective depth d, far away from the loading plate center) leads to a reduction of shear strength with increasing pre-crack width, and the maximum reduction is 21.4%. Based on the detailed analysis of crack propagation, it was clarified that pre-crack may result in two patterns of critical diagonal crack, and in the condition that the pre-crack plane vertically intersects with the lateral splitting part of diagonal crack, the lateral splitting part becomes more severe and thereby reduces the shear strength and deformation ability.

Nondestructive analysis of tritium (T) distribution was performed by means of imaging plate technique on specimens cut from the Be limiters, W-coated carbon tiles and bulk W lamellae retrieved from the JET tokamak after the first and third experimental campaigns with the ITER-like wall. Afterwards, analyses were continued using X-ray photoelectron spectroscopy, microscopy techniques and thermal desorption spectroscopy. Co-deposits formed on the W-coated tiles in the 1st campaign showed large T retention because of high carbon content reaching up to 50 atomic %, while the carbon fraction in co-deposits after the 3rd campaign was distinctly lower. The T retention of the plasma-facing surface of the bulk W tile was smaller than that of the W-coated tiles by a factor of 20, while deposition of small amount of T was found at the side surfaces facing to the gaps in a lamella structure. The correlation of T distributions with surface morphology and the discharge conditions is discussed.